1. Field of the Invention
Embodiments of the invention relate to methods of making extrusion dies and more particularly to methods of making extrusion dies utilizing a LIGA process which are useful for forming arbitrary cellular structures.
2. Technical Background
Conventional extrusion dies for forming cellular ceramic products typically have a series of holes on one side of a metal plate intersecting a series of slots, forming pins, which can have various geometric cross sections such as squares, hexagons, or the like on the opposite side of the metal plate. In order to form the cellular ceramic products, raw materials, i.e. batch, is pushed into the hole side of the metal plate and exits the slot side of the metal plate, forming the cellular shape by being extruded through the slots and around the pins.
A similar process can be used to fabricate, for example, optical fiber by extrusion through an extrusion die. In this process, the fiber can be obtained by a draw-down process from a cane which is extruded through a larger size die. The cane and thus fiber cross-section can be cellular and is known as a photonic band-gap design.
A variety of conventional machining methods can be used to create this tooling geometry, depending on dimensions and design of the extrusion dies. For the machining of the holes, drilling, twist drilling, gun drilling and electrochemical machining (ECM) are typically used. For the machining of the slots, abrasive wheel grinding, wire electrical discharge machining (wEDM), wheel slitting and/or plunge electrical discharge machining (pEDM) are typically used. Typically after machining, the surfaces of the extrusion dies are coated with a wear resistant material to improve the life of the extrusion dies.
The conventional machining methods used for making the extrusion dies have several disadvantages, for example, lack of precision of the dimensions of the holes and/or the slots and the increased fabrication time of the extrusion dies. wEDM and pEDM methods can be limited by the precision capabilities of the electrode and also by the constant wear to the electrode realized through the cutting process. Further, wEDM and pEDM are inherently slow when high precision is required, because dimensional tolerance is inversely proportional to the feed speed. In order to reduce these effects, a second, finishing machining pass is required, further slowing the overall fabrication time of the extrusion dies dramatically. In the case of pEDM, the second finishing machining pass usually requires the use of a second electrode, which also increases the fabrication time and costs.
Moreover, the abovementioned methods cannot always be used to make increasingly intricate patterns, irrespective of the precision or speed. For example, a wEDM method could be used to make a fully intersecting pattern made of straight lines, but not a fully intersecting pattern with, for example, hexagonal cells (honeycomb). pEDM has the disadvantage of requiring an electrode to be machined prior to the cutting process. The electrode is typically made using conventional machining (milling) methods or wEDM, which have well known limitations in terms of precision on features below 2 thousandths of an inch in size. Conventional mill and wEDM machining methods have the inability to produce a zero radius of curvature when a finite diameter bit or wire is used which adds to the precision issue in some of the EDM processes.
Similar disadvantages exist in the fabrication of extrusion dies for optical fiber having special cross-sectional designs. In optical fiber fabrication, a cane is not necessarily a cellular shape, but can be a solid shape having a unique cross-sectional geometry. The optical fiber can be drawn-down from the cane. Alternatively, the cane can be extruded directly at the final diameter, in the 100-1000 microns range through an extrusion die. Such small dimensions combined with a complex geometry make the conventional machining methods unusable.
It would be advantageous to have a method for making an extrusion die which possesses high precision of features. It would also be advantageous to have the method applicable to several different slot geometries depending on the intended application. Further, it would be advantageous for the method to be cost effective and reduce manufacturing time.